use toPlainText() to retrieve value of text field

[bleything]

Description

Resolves #109 using @bcongdon's suggestion of toPlainText(). I couldn't find any tests that cover the viewer so I manually confirmed that both free text and choices type params live update and save/load correctly. Let me know if I'm just missing the tests.

Checklist

• [X] feature/fix implemented
• [X] mypy returns no error
• [X] tests added/updated and pytest --runslow succeeds
• [ ] documentation added/updated and building with no error (make clean && make html in docs/)
• [ ] examples added/updated
• [X] code formatting ok (black and isort)